1. Field of the Invention
This invention relates to very high strength cords composed of a core and a sheath of twisted yarns plied around the core in such a way that the cord exhibits greatly improved retained strength after use. The twisted yarns are generally made from aramid fibers; and are usually made from para-aramid fibers.
2. Description of the Prior Art
U.S. Pat. No. 4,392,341, issued July 12, 1983 on the application of Grill, discloses an apparatus and process for twisting several yarns and plying them to make cords. The apparatus is said to be specially suited to use with aramid yarns and utilizes a plate with equidistant holes for each yarn to serve as a thread guide. There is no teaching of a core/sheath structure.
U.S. Pat. No. 2,882,675, issued Apr. 21, 1959 on the application of Tingas, discloses a device for twisting and plying yarns to make cords. There is disclosure of a guide plate having several holes equidistant from one another and having a hole in the center. There is no disclosure of plying several yarns about a central yarn.
U.S. Pat. No. 3,481,134 issued Dec. 2, 1969, on the application of Whewell discloses a process for eliminating kinks in a core/sheath cord structure by means of twisting the core yarn in an opposite direction from the twist of the sheath yarns and twisting the overall cord structure in the same direction as the core yarn. The reference is directed to multi-ply cords having a core the same size as the yarns of the sheath. The relationship between core and ply yarns is completely outside the formulae of the present invention.
U.S. Pat. No. 4,176,705 issued Dec. 4, 1979 on the application of Russell et al., discloses a composite cord having a core of aramid wrapped by six steel strands. The steel strands are said to be slightly smaller than the aramid core so that the steel strands will be held slightly apart. The core is aramid because it has a load carrying tensile strength.
U.S. Pat. No. 2,755,214 issued July 17, 1956, on the application of Lyons et al., discloses preparation of cords having a nylon or polyester core with a sheath of twisted low modulus rayon yarns about the core. This reference is devoted to improving the creep character of low modulus rayon yarns and there is no recognition of any loss of cord strength due to compression fatigue in high modulus yarns.